The present invention generally relates to the melting of scrap metal in contact with molten metal, and in particular, to the melting of solid aluminum scrap in a continuous process.
In the melting of solid scrap metal, such as, for example, aluminum, the solid material is desirably heated as rapidly as possible to achieve the highest possible melting rate. In current practice, the solid metal is introduced to a molten body of metal within a furnace generally in a ratio of about 2 pounds of solid material to 1 pound of molten metal, after which heat is applied to the mixture conventionally from above, to melt the solid component.
In the above practice, many drawbacks exist that prevent rapid heat input to the solid charge of metal. Specifically, it is difficult to rapidly transfer heat into solid pieces of metal without producing high surface temperatures therein, resulting in accelerated oxidation and a substantial oxide skim which floats on the melt and insulates the melt from the transfer of further heat. Also, a high percentage of melt loss occurs with scrap comprising finely divided material, which results from the large surface area of such material.
An additional problem which occurs with metals such as aluminum, is present when the metal is heated in a relatively deep bath. In spite of the high thermal conductivity of aluminum, heat loss from the bath through the furnace wall containing the bath creates temperature gradients or thermal stratification in which the surface temperature can be as high as 100.degree. F greater than the metal temperature at the bottom of the bath. Such high temperature causes rapid oxidation which, as noted above, serves to insulate the melt and further reduces melting rate while increasing melt loss. The inability to efficiently distribute heat in the furnace results in the build up of temperatures up to 3,000.degree. F or greater above the metal level which causes the furnace linings to deteriorate and necessitates the reduction of heat input.
It has been proposed to employ separate metal charging and melting bays or compartments connected by a loop through which molten metal is circulated. In this technique, the improved heat transfer between the moving molten metal and the solid charge pieces is utilized. Heat is applied to the molten metal in one compartment so that it can be transferred to the solid charge in the other compartment. Even these techniques, however, have had limited success, as, when heat is not efficiently utilized, the same problems result which were discussed above in connection with single charging furnaces.
One solution which has been proposed to the problems encountered in the separate charging and melting technique, is to maintain a melt body at a depth of from 1 to 31/2 feet whereby heat is applied to the upper melt surface while a cooled molten metal stream is continuously introduced across the bottom portion of the melt and upwardly through an arc to effect a sweep-flow reversal of the upper melt region. At the same time, a heated stream of metal is continuously removed from the upper regions and introduced into a charging receptacle, where solid metal is introduced at a weight ratio of at least one part solid metal to at least 10 parts molten metal. This technique is fully discussed in U.S. Pat. No. 3,770,420 to Spear et al.
The present invention provides a different approach to the problem discussed above than that taken by Spear et al., and is believed to result in a greater economy and efficiency of operation.